1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device used for a DC-DC converter, for example.
2. Background Art
Recently, with the decreasing voltage of power supplies used for the CPU (Central Processing Unit) of computers, for example, power supplies based on synchronous rectification (e.g., step-down DC-DC converters) are widely used. Furthermore, the current change rate (di/dt) required for a CPU power supply is ever increasing, and the ripple of the output voltage of the power supply needs to be held down. To this end, increasing the switching frequency of the power supply becomes important. In addition, with the decrease of the CPU operating voltage, the CPU power supply is required to provide low voltage and large current. Thus it is desired to increase the switching frequency and efficiency of the power supply system (DC-DC converter) itself that supplies electric power.
In a step-down DC-DC converter, a MOSFET is used as a switching (chopping) element (see, e.g., JP-A 2004-511910(Kokai)). In an on-chip DC-DC converter with built-in switching elements, if the operating current is increased, it is difficult to uniformly turn on/off the entirety of a plurality of switching elements due to the effect of interconnect resistance. In an on-chip output element, the gate-source breakdown voltage is often set lower than the drain-source breakdown voltage. Thus a driver power supply line for turning on the gate of the output element is needed besides the input voltage line and the ground line. The power supply line of a driver circuit for driving this switching element is often formed thinner and longer than the input voltage line of the converter and the ground line, and is particularly susceptible to parasitic interconnect resistance. When the switching elements are driven, the switching element located distant from the power supply of the driver circuit cannot rapidly charge/discharge the power supply line, increasing the switching time and loss at this location. Furthermore, the switching element located nearer to the power supply of the driver circuit switches earlier, whereas the switching element located distant therefrom switches later, causing current variation in the semiconductor substrate. Concentration of current on one location may lead to device destruction.
On the other hand, when a low-side built-in diode is turned on during the dead time of the DC-DC converter, electrons are injected into the semiconductor substrate. The injected electrons flow to electrodes at higher potential than GND (ground), causing malfunctions and latch-ups, which may lead to device destruction.